1. Field of the Invention
The present invention relates in general to a method for recovering metals included in the slag generated from steelmaking process using a convertor, and more particularly to a method for selecting the slag containing high metals content from the relative light slag.
2. Description of the Prior Art
Since a considerable quantity of useful metals remain in slag, which is generated from the steel-making process, a method for recovering the useful metal from the slag is required. Conventionally, for the sake of recovering the useful metals included in the slag, the slag is pulverized in large-sized pulverizers to obtain powder which is subsequently conveyed to a furnace on a conveyer mounted with a magnet.
However, the conventional method costs a great deal since a plurality of large-sized pulverizers and magnetic conveyers which are expensive must be installed to recover the useful metal included in slag. Especially, polluting factor such as dust and noise occurs during the pulverization of slag, so that the working conditions for workers become deteriorated.
In the meanwhile, the slag generated in a convertor during making steel is comprised of various constituents, mainly Fe.sub.2 O.sub.3 and CaO. Of the constituents, materials including Fe, such as FeO and Fe.sub.2 O.sub.3 are extracted from the slag. The slag from which the useful metal is recovered (hereinafter "recovered slag") is then discarded as it is or is used as landfill. However, there occur many problems in the recovered slag since the main constituent, calcium oxide (CaO), is vulnerable to weathering. For example, in case of discarding the recovered slag in the sea, calcium oxide is likely to be changed by seawater and salts into an alkaline compound such as calcium hydroxide (Ca (OH).sub.2) compound, which pollutes the seawater and arrests the growth of fishes and marine plants. In addition, the use of recovered slag as landfill may causes the subsidence of ground. On the other hand, in the case the recovered slag is left as it is on land, the recovered slag is weathered to become powdery, so that alkaline fine powder such as calcium oxide may blow to injure nearby residential streets and forests.
By definition, weathering is the action in that a rock or an ore is broken down and decomposes into earth and sand on the surface on or in the earth. Weathering is largely classified into mechanical weathering (physical weathering) and chemical weathering. Mechanical weathering means that a rock is weathered by mechanical force. For example, as the air is heated and cooled, a rock or the like undergoes repeated expansion and contraction resulting in very small crevices in which freezing, hydrating and salt crystal extracting occurs. The rock ultimately splits due to inner expansion into pieces, loses its crystal lime water finally becomes powder. In addition, a rock is broken down by the mechanical destruction of living things such as microorganisms and plants. On the other hand, chemical weathering, by definition, means that the constituent of a rock, an ore or the like is chemically changed and is discomposed by chemical action of chemical agents or living things, such as hydration, hydrolysis, oxidation, reduction, carbonation, ion exchange and dissolution. Chemical weathering process is generally dependent on the properties of the rock or the ore, pH of solvent such as rainwater, the amount of compound dissolved in the solvent, such as oxygen, carbonic acid gas, ammonia, inorganic acid and organic acid, temperature and the like. By way of example, on the surface of an ore or slag exist unsaturated radicals, which are generally reactive points with water, hydrolysis and hydration occurring thereat. Accordingly, metal ion, M.sup.+ in the ore or the slag becomes MOH, while H.sup.+ if it exists in the slag, permits the ion of crystal to be changed into hydroxide ion (OH-) which combines with CaO, MgO and the like to demolish the existing structure thereof. As a result, the slag is broken down into particles such as clay which is then dehydrated into dry, fine powder. In most cases, the two classes of weathering have influence on an object, simultaneously and synergistically. Consequently, the slag is decomposed by these weathering into a powder of CaCo.sub.3, CaO, Ca(OH).sub.2, MgO, Fe.sub.2 O.sub.3, Fe(OH).sub.3 or the mixture thereof, which then blows into the air and causes serious pollution.